I have a five year old home. The sewer smell is coming from the basement furnace room. Near the water heater is a hole in the concrete floor (for a sump pump). A sump pump was never installed and now is loosley covered with plywood. During extremely cold weather there is a strong sewage order from this hole which circulates throughout the house when the heat is on. The home has a septic tank. Is it possible the sump pump hole is somehow connected with the septic system?